Broadband microstrip hybrid tee



Sept. 22, 1970 Filed Dec. 16, 1968 R. E. BLIGHT BROADBAND MICROSTRIP HYBRID TEE .4 Sheets-Sheet l Sept. 22, 1970 R. E. BLIGHT 3,530,407

BROADBAND MICROSTRIP HYBRID TEE Filed Dec. 16, 1968 4 Sheets-Sheet :3

RONALD E. BLIGHT, INVENTOR TTORNEYS Sept. 22, 1970 R. E. BLIGHT BROADBAND MICROSTRIP HYBRID TEE .4 Sheets-Sheet 5 Filed Dec. 16, 1968 Sept. 22, 1970 R. E. BLIGHT 3,530,407

BROADBAND MICROSTRIP HYBRID TEE Filed Dec. 16, 1968 .4 Sheets-Sheet 4 FIG. 6

RONALD BLIGHT, INVENTOR ORNEYS United States Patent 3,530,407 BROADBAND NHCROSTRIP HYBRID TEE Ronald E. Blight, Framingham, Mass., assignor to Microwave Associates, Inc, Burlington, Mass., a corporation of Massachusetts Filed Dec. 16, 1968, Ser. No. 784,148 Int. Cl. H01p 5/12 U.S. Cl. 333-11 23 Claims ABSTRACT OF THE DISCLOSURE A broadband microstrip hybrid tee in one layer pattern above a ground plane is disclosed. Four ports are constructed and connected in a manner such that a signal feed into one of the branch arms will divide equally and in phase between two arms connected to matched loads with no signal appearing in the fourth branch. Similarly a signal fed into the fourth branch will divide equally but out of phase between the two arms connected to matched loads, and no signal will appear in the first branch arm.

BACKGROUND OF THE INVENTION Recent developments in microwave techniques have brought to the forefront microstrip transmission line has a suitable means for fabricating integrated circuitry. (Microstrip is defined as a conductive pattern on a dielectric sheet which has a ground plane on its opposite face.) With the use of relatively high dielectric constant substrates such as alumina ceramics and single crystal alumina (sapphire) a substantial reduction in physical line lengths is possible and a means of economic reproducibility is provided by the simplicity of this microstrip form. Circuits with active devices such as transistors and avalanche oscillators that call for both DC. and high frequency circuits, and any number of bias resistors, capacitors, couplers, and decouplers, etc. can be economically fabricated closely spaced but electrically decoupled on a single chip. Microstrip has been found suitable for moderate, and low power microwave components ranging in frequency from UHF to K-band.

Microstrip is essentially a single plane or two dimensional structure whereas waveguide is essentially a three dimensional structure. Stripline on the other hand differs from both waveguide and microstrip in that it is essentially a one layer pattern circuit sandwiched between ground planes. The components or structures of one type system such as couplers, transformers, tees, and junctions cannot, in general, be adopted for the needs of the new technology. New structures, new materials, and new concepts are in general necessary to perform old tasks. For example: a rectangular waveguide magic tee or hybrid tee is constructed by connecting waveguides in shunt and in series with a collinear guide at the same point. This structure is three dimensional for in essence an H-plane or shunt tee has been connected at right angles to an E-plane or series tee. This structure has some useful and unique properties. If a microwave signal is fed in the shunt arm, the power will divide equally and in phase between the two collinear arms at points equidistant from the junction, if they are terminated in matched loads, no power will appear in the series arm. If power is fed into the series arm, power will divide equally but out of phase between the two collinear arms if they are terminated in matched loads and no power will appear at the shunt arm. These properties have several uses. One use is in balanced mixers. By terminating the collinear arms with matched mixer diodes and feeding an incoming signal in the shunt arm, and a local oscillator signal in the series arm, the two signals can be mixed resulting in an intermediate frequency but without local oscillator noise present. An IF signal resulting from the mixing of the signal and local oscillator in the two diodes is filtered out and combined in a lower frequency signal. This results in balanced mixers with lower overall noise figures. Similarly, it is desirable to have balanced mixers (as well as many other devices now feasible within the technology of waveguides, coaxial lines, and Stripline) in microstrip. A broadband microstrip hybrid tee (the counter-part of a waveguide magic tee) among other uses, facilitates for one, the fabrication of balanced microwave mixers on microstrip. Although, in essence it is required to connect electrically two transmission lines in shunt and in series there are problems to be overcome in microstrip because of its size, two dimensional characteristics, and the fabricating materials available; for one, a new concept is required to interconnect the series and parallel circuit and still maintain a broadband devicethis is hybrid microstrip on microstrip. Another requirement is that of obtaining equal electrical lengths with physically unequal lengths of transmission lines overlaid one on top of another. Still another is the requirement of transforming from microstrip to other type connections without destroying broadband characteristics. These and other problems dictated a unique device as set forth in this invention.

SUMMARY OF THE INVENTION This invention relates in general to hybrid tees and in particular to broadband microstrip hybrid tees.

A one layer pattern of electrical conductor is deposited on one face of a dielectric with dielectric constant greater than one whereas a continuous electrically conductive layer to act as ground plane is deposited on the other face of the dielectric. Standard photoresist, masking, evaporation and thin film depositing techniques are utilized. On a portion of the deposited circuit, the conductor is overlaid with an additional dielectric 'which in turn is overlaid by a fine electrically conductive wire having a diameter very much smaller than the width of the conductor beneath it. Standard ultrasonic bonding techniques familiar in semiconductor techniques are used.

One feature of this invention is that it permits the fabrication of a circuit on microstrip which is almost the electrical equivalent of the well-known waveguide magic tee.

Another feature of the invention is that it is a broadband device with one embodiment of the invention having bandwidth.

Still another feature of the invention is its reduced size Which permits its use in microwave integrated circuitry.

DESCRIPTION OF THE INVENTION Exemplary embodiments of the invention and methods to make them are described with reference to the accompanying drawings, not to scale, in which:

FIG.1 illustrates schematically the essential elements of a structure in accordance with this invention.

FIG. 2. illustrates schematically a cross section AA of a portion of the circuit of FIG. 1.

FIGS. 3A and 3B illustrate schematically the metalized aperture, physical length and transformation unit.

FIG. 4 illustrates schematically another embodiment in accordance with this invention.

FIG. 5 illustrates schematically cross section B--B of a portion of the circuit of FIG. 4.

FIG. 6 is a plan view of FIG. 1, showing case and connectors.

In FIG. 1, a dielectric substrate of alumina 5, although any material with dielectric constant greater than 1 could be used such as polystyrene, steatite, sapphire, titanium dioxide or even semiconductor material such as silicon, germanium or gallium arsenide, is metalized on two of its faces 6 and 9 whereon a conductive pattern is formed on face 9 by etching away unwanted metal to expose the substrate below. Apertures 7 and 8 are also metalized. (See FIG. 3 which shows apertures 7 and 8 completely metalized within its exposed walls 20, 21, 22, and 23.) This metalizing can be a thin layer of chrome and then a layer of gold, although any other electrical conductor compatible with the substrate may be used. A typical method of applying the conductive layers is to vacuum evaporate on one face at a time on the substrate heated to a temperature greater than 400 C., a splash of chrome and then gold, in order to form a good bond between the conductor and the substrate. Gold is then electroplated to a thickness consistent with microwave energy penetration at a given frequency when this energy propagates therein. For X-band at least 0.3 mil thickness of conductive layer would be sufficient, and for lower frequency bands a thicker conductor would be necessary since skin epth of the radiation is greater. Substrate thickness is of the order of 30 mils, while length and width are about /2 inch on each side.

By utilizing standard photographic, masking and photoresist techniques well known in the semiconductor and integrated circuit industry, a circuit as shown in FIG. 1 is formed bonded to the substrate 5. Adding overlay dielectric 25 over conductors 13 and 14 and a thin overlay conductor 10, over the conductors 13 and 14 and over the overlay dielectric, completes the circuit. The overlay dielectric material 25 may be, for example, polystyrene.

Referring to FIG. 1, the circuit comprises an input port 3 electrically connected to two branching arms, known in microwave parlance as a 3 db power divider. Each leg 11 and 12 of this power divider is one quarter of a wavelength long at its band center. Leg 11 is electrically connected to leg 16 which in turn terminates in port 2, while leg 12 is electrically connected to leg 15 which in turn terminates in port 1. Each quarter wavelength leg 11 and 12 is approximately 709 so that with ports 1 and 2 terminated by 509 loads, port 3 would also see 509. Legs 13 and 14 are also one quarter wavelength conductors electrically connected to legs 11 and 12 at points 17 and 18 respectively at one end and to the metalized walls of apertures 7 and 8 respectively at their other end and their impedance to ground is in the order of 1259 each for broadband performance.

With the addition of overlay dielectric 25 (see FIG. 2) not necessarily and preferably not of the same dielectric constant and overlay conductor 10, to this circuit a transmission line is formed between first conductors 13 and 14 and overlay conductor 10 where conductors 13 and 14 now act as ground plane for conductor 10. This transmission line is above ground (see FIG. 2). Beyond apertures 7 and 8 (FIG. 1), a conductor 10 transforms at both ends to conductors 40 and 50, one of which, namely conductor 50, is connected to port 4 of the device and the other conductor 40 is not terminated and left open circuit. The electrical line length from points 140 to 150 is one-half wavelength at the center frequency. The construction of this transmission line is made clearer by referring to FIG. 2 which represents a cross section taken along A-A on FIG. 1. Referring to FIG. 2 and also to FIGS. 3A and 3B, a dielectric substrate has a conductive metalized face 6 which acts as a ground plane; on its opposite face, a first electrical conductor 14 is overlaid by a dielectric 25, which in turn is overlaid by a small diameter electrical conductive wire (Although FIG. 2 does not show it, conductor 13 is also overlaid by a dielectric.) The spacing, dimensions, configuration, and choice of materia 9 b trate,

first conductor, second conductor and overlay dielectric are critical in the proper operation of this device as a broadband component. For example, in FIG. 1, in order to contain the incoming signal along its intended path from port 3, to ports 1 and 2 without waveloss or reflection over the components entire operating band, the impedance to ground of conductors 13 and 14 respectively must be as high as other conditions permit. Thus for high impedance between ground plane 6 and first conductors 13 and 14, the substrate between ground plane and conductors must be as thick as can be tolerated, and the conductor width must be as small as possible. On the other hand, the spacing between first conductors 13 and 14 and and overlay conductor 10 must be as small as possible so that first conductors 13 and 14 form an effective shield for the microwave energy propagating between first conductors 13 and 14 and second conductor 10, and no energy or very little energy propagates between second conductor 10 and ground plane 6-. These conditions dictate an exceedingly small cross section for uppermost conductor 10, since conductors 13 and 14 must be small in width to fulfill one set of conditions, as discussed above. Furthermore since the dielectric constant between first conductors 13 and 14 and ground plane, and the dielectric constant between first conductors 13 and 14 and second conductor 10 is different, the velocity of propagation will be different and this results in different physical line lengths for the same electrical line lengths. This physical difference is accomplished in part by interposing metalized apertures 7 and 8 which electrically connect conductors 13 and 14 to ground plane via section 26. Hence, first conductors 13 and 14 are terminated to ground at their respective ends whereas uppermost conductor 10 is not terminated to ground being open circuit at one end and connected to metal conductor 50 forming input port 4 at the other. The combination of uppermost conductor 10 and continuation conductors 40 and 50 provide a desired electrical length used for impedance matching.

FIG. 4 schematically represents another embodiment of this invention. Conductors 122 and 123 are separated by a dielectric and form a series resonant circuit at the center frequency. Details of this construction are schematically shown in FIG. 5, where 106 is an electrical conductor which acts as ground plane, is a substrate dielec trio with dielectric constant greater than 1, 123 is a first conductor above the ground plane 106, 122 is a second conductor above a ground plane and overlaying the first conductor, and 109 is a dielectric between and separating first conductor from second conductor. Note that the uppermost conductor 122 is not a small diameter wire but rather a deposited metallic structure rectangular in shape.

Referring to FIG. 4 again, conductor 113 is connected to 122, 112, and 111 at point 126 and is electrically connected to ground plane 106 through metalized walls 119 of a structural type previously described and constitutes first conductor above ground plane. (Details of the metalized hole or aperture connection are shown schematically in FIGS. 3A and 3B.) Conductor 111 is connected to 116 at point 117 and conductor 112 is connected to 115 at point 118. Points 117 and 118 are similar to points 17 and 18 respectively of FIG. 1. The entire configuration is overlaid with a thin dielectric such as 0.2 mil polystyrene or quartz, although only conductors 111, 112, and 113 absolutely require the dielectric to electrically separate the second conductor above the ground plane, from the first conductors 111, 112, and 113 above the ground plane 106.

In operation, a signal entering port 103 splits equally and in phase at point 126, whereas energy entering port 104 is carried between upper two conductors effectively shielded from the ground plane, and splits equally and out of phase into conductive arms 111 and 112. The series element composed of conductors 122 and 123 is a broadband matching feature because it provides a reactance versus frequency characteristic that is used in conjunction with the other elements of the combination to maintain a broadband resistive impedance. For example, as frequency changes from center frequency a reactive component of impedance appears at 124 due to other elements being now non-resonant; the series element composed of conductors 122 and 123 now provides an equal and opposite reactance because it too is non-resonant. Port 103 then effectively sees no substantial reactive component of mismatch, hence broadband characteristics are not lost. In all other aspects of operation, such as power split by symmetry, etc., this configuration is essentially the same as the previously described embodiment. Whereas previous embodiment used two apertures 7 and 8, here only one is used. The transmission line above ground formed between conductors 113 and 110 (FIG. 4) beyond cross over point 117 and 118 progressing in an anti-clockwise direction remains a transmission above ground during its entire electrical length to the open circuit end.

FIG. 6 is a plan view of FIG. 1 showing the invention inclosed in a standard microwave enclosure 500 having standard coaxial connectors 600, 601, 602, 603 connected to ports 3, 4, 1, and 2 respectively.

The embodiments of the invention which have been illustrated and described herein are but a few illustrations of the invention. Other embodiments and modifications will occur to those skilled in the art. No attempt has been made to illustrate all possible embodiments of the invention, but rather only to illustrate its principles and the best manner presently known to practice it. Therefore, while certain specific embodiments have been described as illustrative of the invention, such other forms as would occur to one skilled in this art on a reading of the foregoing specification are also within the spirit and scope of the invention.

What is claimed is:

1. A high frequency electrical network comprising a first electrically conductive ground plane, first, second and third ports above the first ground plane, a 3 db power divider having three electrically conductive arms above the first ground plane with one of its arms connected to the first port, first and second electrical junctions above the ground plane with the second arm of the power divider connected to the first junction and with the third arm of the power divider connected to the second junction, first conductive means providing a first wave path above the first ground plane between the first junction and second port, second conductive means providing a second Wave path above the first ground plane between second junction and third port, the combination of such electrical circuits forming a first network of electric wave paths above the first ground plane so that electromagnetic energy entering the first port splits equally and in phase by symmetry at the power divider and becomes available at the second and third ports; a second ground plane above the first ground plane, a fourth port above the second ground plane, and electrical conductive means providing an electrical path above the second ground plane between the fourth port and first electrical network at the first and sec ond junctions so that energy entering the fourth port splits equally but out of phase at the first and second junctions and is conducted to second and third ports on first and sec ond conductive means respectively.

2. A high frequency electrical network as recited in claim 1 wherein said first network of electric wave paths comprises a first printed circuit in one layer pattern above said first ground plane.

3. A high frequency electrical network as recited in claim 2 wherein said first printed circuit is fabricated on a dielectric material with dielectric constant at least twice that of air.

4. A high frequency electrical network comprising a ground plane, a first port for the introduction or withdrawal of electromagnetic wave energy therein, a 3 db power divider connected at its first arm to said first port and having its second and third arms one quarter of a wavelength long, a second and third port for the introduction of Withdrawal of electromagnetic wave energy therein, fourth and fifth arms with said fourth arm connected at one end to said second arm and its other end connected to said second port while said fifth arm is connected at one end to said third arm and its other end connected to said third port, a sixth and seventh arm each one quater wavelength long said sixth arm connected at one end to the ground plane and at the other end connected to said second and fourth arm at their junction while said seventh arm is connected at one end to said ground plane and at its other end to said third and fifth arms at their junction, a fourth port for the introduction or withdrawal of electromagnetic wave energy, and an overlay conductor running the combined lengths of said sixth and seventh arms disposed above and physically separated from said sixth and seventh arms which arms shield said overlay conductor from said ground plane, and having one end of said overlay conductor connected to said fourth port whereas the other end of said overlay conductor terminates in an open circuit.

5. A high frequency electric network as recited in claim 4 wherein that portion of circuit comprising said overlay conductor and said sixth arm, and said overlay conductor and said seventh arm is one half of a wavelength long.

6. A high frequency electrical network as recited in claim 4 wherein said overlay conductor, running the combined lengths of said sixth and seventh arms disposed above and physically separated from said sixth and seventh arms, is separated from said sixth and seventh arms by a dielectric material with a dielectric constant between 7 and 11.

7. A high frequency electrical network comprising means providing a first ground plane, first, second and third ports above said means providing a first ground plane, a first electromagnetic wave path means disposed above said means providing a first ground plane electrically connecting said first port with said second and third ports for conducting electromagnetic wave energy from said first port to said second and third ports, said electromagnetic wave path means and said first, second and third ports comprising a first electrical network, means providing a second ground plane disposed above said means providing a first ground plane substantially in the same plane as said first electrical network and electrically connected at selected regions to said means providing a first ground plane, a fourth port above said means providing a second ground plane and electromagnetic wave path means above said means providing a second ground plane, providing an electromagnetic wave path above said means providing a second ground plane between said fourth port and said first electrical network for conducting electromagnetic wave energy from said fourth port to said second and third ports.

8. A high frequency electrical network as recited in claim 7 wherein said first electrical network comprises a printed circuit in one layer pattern above said first ground plane.

9. A high frequency electrical network as recited in claim 8 wherein said first electrical network is fabricated on a dielectric material having a dielectric constant at least twice that of air.

10. A high frequency electrical network as recited in claim 9 wherein said dielectric material is selected from the group consisting of alumina, steatite, sapphire, titaniurn dioxide and beryllia ceramic.

11. A high frequency electrical network as recited in claim 8 wherein said printed circuit is fabricated on semiconductor material.

12. A high frequency electrical network as recited in claim 11 wherein said semiconductor material is selected from the group consisting of germanium, silicon and gallium arsenide.

13. A high frequency electrical network comprising a ground plane, a first port for the introduction or withdrawal of electromagnetic wave energy therein, first and second electrical conductors separated by a dielectric and disposed spatially above one another and above said ground plane and forming a first electromagnetic wave path guide for electromagnetic wave energy transmission therethrough said electromagnetic wave path guide being electrically connected at one of its ends to said first port, a 3 db power divider having first, second and third arms and electrically connected at said first arm to the other end of said electromagnetic wave path guide and having said second and third arms one quarter of a wavelength long respectively, a second and third port for the introduction or withdrawal of electromagnetic wave energy therein, fourth and fifth arms with said fourth arm electrically connected at one end of said second arm and at its other end electrically connected to said second port while said fifth arm is electrically connected at one end to said third arm and at its other end electrically connected to said third port, said power divider and said first, second, third, fourth and fifth arms forming a first electrical network above said ground plane electrically connected to said first electromagnetic wave path guide for transmitting electromagnetic wave energy from said first port to said second and third ports; a fourth port for the introduction or withdrawal of electromagnetic wave energy; a sixth arm one quarter wavelength long said sixth arm electrically connected at one end to said ground plane and at its other end electrically connected to said power divider at said first arm, and an overlay conductor disposed spatially above and running the length of said sixth, second and third arms said overlay conductor forming a second electromagnetic wavepath guide with said sixth, second and third arms said second electromagnetic wavepath guide being electrically connected to said fourth port at one end and being electrically connected at its other end to said first electrical network for transmitting electromagnetic wave energy from said fourth port to said second and third ports.

14. A high frequency electrical network comprising a first ground plane, first, second and third ports above said first ground plane, a power divider disposed above said first ground plane electrically connecting said first port with said second and third ports, said power divider and said first, second and third ports comprising a first electrical network separated from said first ground plane by a first dielectric material said first electrical network capable of transmitting in-phase electromagentic wave energy from said first port to said second and third ports; a second ground plane spatially above said first ground plane and substantially in the same plane as said first electrical network, a fourth port above said second ground plane, and an electrical conductor disposed above 8 said second ground plane and separated from said second ground plane by a second dielectric material said electrical conductor electrically connecting said fourth port and said first electrical network providing an electromagnetic wave path above said second ground plane from said fourth port to said first electrical network, with electro magnetic wave energy entering said fourth port being conducted in out of phase relationship to said second and third ports.

15. A high frequency electrical network as recited in claim 14 wherein said first electrical network comprises a first printed circuit in one leyer pattern above said first ground plane.

16. A high frequency electrical network as recited in claim 15 wherein said electrical conductor comprises a second printed circuit in one layer pattern above said second ground plane.

17. A high frequency electrical network as recited in claim 14 wherein said first dielectric material has a dielectric constant at least twice that of air.

18. A high frequency electrical network as recited in claim 17 wherein said second dielectric material has a dielectric constant between 9 and 11.

19. A high frequency electrical network as recited in claim 14 wherein said first dielectric material is selected from the group consisting of alumina, steatite, sapphire, titanium dioxide and beryllia ceramic.

20. A high frequency electrical network as recited in claim 19 wherein said second dielectric material is polystyrene.

21. A high frequency electrical network as recited in claim 14 wherein said first dielectric material is a semiconductor material.

22. A high frequency electrical network as recited in claim 21 wherein said semiconductor material is selected from the group consisting of silicon, germanium and gallium arsenide.

23. A high frequency electrical network as recited in claim 20 including aperture means having metallized walls in said first dielectric for electrically connecting said first ground plane with said second ground plane.

References Cited UNITED STATES PATENTS 2,836,798 5/1958 Le Vine 333 l1 X 3,164,791 l/l965 Dent 3331l 3,237,130 2/1966 COhn 333-10 PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 333-84' 

